An American named Frederick Henry, who is studying architecture in Rome, undergoes a transformation when World War I breaks out. He becomes a Lt. in the Italian Army. Even though America doesn't enter the Great War for another 3 years, he volunteers. Why? Perhaps he himself can't quite say, but young men often seek excitement in their otherwise dull lives. He joins the ambulance corps on the northern front, in charge of four drivers and a few motorcars. Their task is to pick up the badly wounded soldiers. When possible, the dead are carried outside the vehicles as there is no need for them inside. The Austrians are the enemy, but the high snowy mountains and freezing weather make the battles extremely difficult. The swollen rivers are dangerous to cross, the artillery flashes in the night, and the screaming mortars fly above, coming down no one knows where, except for the unfortunate victims, and it's too late for them.
Rinaldi, a very capable surgeon in the Italian army, is getting better every day. He is constantly putting back together the wounded bodies and saving lives most of the time. He is Lt. Henry's affable roommate, always joking and his best friend. They live in a good house in a mostly undamaged village near the war, for officers. Rinaldi loves women to a certain degree (lust might be the more accurate word). He has seen his latest enchanting female, but to his deep regret, there is no mutual feeling between them. The gracious doctor tells the lieutenant about the beautiful blonde, tall British nurse, Catherine Barkley, and even introduces him. It doesn't take long for a romance to blossom. She lost her fiancee in France in 1916, in the trenches. At first, she, and then he too, falls in love, not wanting or expecting it. Her best friend and fellow nurse, Helen Ferguson, disapproves. Lonely people amid a terrible conflict somehow require something to continue their joyless existence.
Shortly after, while waiting in a ditch at the front for the bombardment to stop, a mortar shell hits. It kills one of his men and badly wounds him in both legs. This time, the ambulance will take the driver for a ride not in the front but in the back. The young American feels a warm liquid dropping from the top. The blood oozing out of another soldier will not stop. Henry can't move, he can only endure until there is no more. The vehicle stops traveling as heavy rains pour down. The dead man is put on the muddy ground, and another victim is carried inside. They finally arrive at the unsanitary field hospital, having safely navigated the treacherous mountain roads and bombs. Catherine becomes the Lt.'s nurse and much more. Since Milan is not far away and an American hospital has just opened (this is 1917), it is a better place for treatment. Catherine gets assigned there, and she says mysteriously that it is never a difficulty. But the recovered Mr. Henry must go back to the front when he is healed, and their happiness is over. This novel is based on Hemingway's experiences in the war. He was a 19-year-old ambulance driver, almost dying of battle wounds and having an unhappy affair with an older nurse.